Time and traffic controlled trunking system



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TIME AND TRAFFIC CONTROLLED TRUNKING SYSTEM f 14 Sheets-Sheet 14 Nov. 6, 1956 Filed Jan. 8, 1954 ATTORNEY United States Patent TIME AND TRAFFIC CONTROLLED TRUNKING SYSTEM Charles O. Parks, East Norwalk, Conn., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 8, 1954, Serial No. 402,963

9 Claims. (Cl. 179-18) This invention relates to communication systems and particularly to trunking arrangements and supervisory signaling over trunks between telephone oces.

Objects of the invention are more flexible trunking ar rangements to automatically care for variations in traic over interothce trunks between telephone oces and a more ecient use of such trunks for supervisory signaling in connection with these trunks.

A feature of the invention is a telephone system comprising a group of two-way interoice trunks arranged for one-way operation in either direction, the number of trunks available for use in each direction being automatically varied according to the time of day so as to provide for average time variations in trafc between two oliices.

Another feature is a telephone system according to the preceding paragraph in which trunks are automatically changed from one direction of operation to the other independent of the time of day control whenever there are more calls in a given direction than there are trunks then arranged for operation in that direction. Y

A further feature of the invention is a telephone system in which the signaling channel of one of the trunks in a` group of two-way interoice trunks, each of which is arranged for operation as a one-Way trunk in either direction, is used for controlling the automatic transfer of trunks from one direction to the other depending on relative tra'ic in the two directions.

These and other features of the invention as embodied in an automatic telephone system are shown schematically inthe drawings and described in this specication. The invention is, however, not limited to the specific system and arrangements shown in the drawings but may be applied generally tov all telephone systems comprising interoi`ce trunks.

Referring to the drawing:

Figs. 1 and 2 show two toll switching o'ices A and B, each comprising crossbar switching equipment and control apparatus for selecting interoflice trunks between these two ofces, two reversible trunk circuits, one in each otice, these trunk circuits being interconnected by a composite signaling channel and by two voice transmitting channels, one in each direction, and trunk direction reversing and supervisory control circuits, one in each oce, the two control circuits being operatively interconnected by composite signaling channels;

Figs. 3 and 4 show one of the reversible trunk circuits in detail;

Figs. 5 to 9, inclusive, show a group of reversible trunk circuits and the reversing and supervisory control circuit equipment in oftice A;

Figs. to 14, inclusive, show a group of reversible trunk circuits and the reversing and supervisory control circuit in oce B; and

Fig. shows the arrangement of Figs. 5 to 14 to form an operative interolice trunking arrangement.

The two toll oces A and B shown in Figs. 1 and 2 include switches of the crossbar type together with control means including senders and markers and connectors for operatively connecting the control means to the switches. Each oce includes incoming trunk circuits such as and 210 connected to trunks from other toll and local otices, outgoing trunks such as 137 and 237 connected to trunks outgoing to other toll and local oces, reversible trunk circuits such as 130 and 230 arranged for use at times as incoming trunk circuits and at other times as outgoing trunk circuits, and direction reversing and supervisory control circuits 140 and 240 which control the direction in which the trunk circuits 130 and 230, respectively, are employed at any particular time. Each of the trunk circuits 110, 130, 137, 210, 230 and 237 is a four-wire trunk circuit, providing a two-wire talking channel in each direction. Crossbar switches such as 111, 112, and 116 in ofhce A and 211, 212, 215 and 216 in oce B are provided for extending connections between incoming trunk circuits and outgoing trunk circuits. The ofces A and B include sender circuits 118 and 218 for use in registering and transmitting oce code and subscriber directory numbers and marker circuits 120 and 220 for controlling operation of the switches. Ofce A also includes connector units 113, 114, 117, 119, '121, 122, 133 and 134 and oice B includes connector units 213, 214, 217, 219, 221, 222, 233 and 234 for operatively connecting the senders and markers with the trunk circuits and switches, as required. The incoming and outgoing trunk circuits 110, 210, 137 and 237, the crossbar switches, the sender and marker circuits, and the connector units are Ashown only in outline form, a complete description of such switches and circuit units being found in Patent 2,23 6,246 granted March 25, 1941, to G. V. King, J. B. McKim and O. Myers and Patent 2,382,893 granted August 14, 1945, to B. McKim and O. Myers. The reversible trunk circuits and 230 are each similar to the trunk circuit TRK101 shown in detail in Figs. 3 vand 4. As shown, these trunk circuits and the associated sender circuits are arranged for multifrequency signaling with respect to theitransmission of rou-ting and directory number digits. For the purpose of this description it is assumed that there are twenty trunks associated with reversible trunk circuits 130 in office A and with reversible trunk circuits 230 in otlice B. This is not to be construed as a limitation on the size of a trunk group to be handled by the invention as trunk group of any size may be so handled. The first and last three of these trunk circuits in oice A, designated TRK101, TRK118, TRK119 and TRK120 are represented in Fig. 6 by the LC, OS and S relays of each; and like representations of the associated four trunk circuits in oce B are found in Fig. 13.

The co-nductors of trunk 50 and repeating coils 152 and 252 provide one talking channel, the conductors of trunk 70, the repeating coils 172 and 272 provide a second talking channel, and the repeating coils 162 and 262 in combination with the conductors of both trunks 50 and 70 provide a third talking channel between offices A and B. These talking channels are associated with reversible trunk circuits 130 and 230, and other trunks, not shown, are provided so that each trunk circuit 130 in ofce A is connected by two talking channels, one in each direction, with each trunk circuit 230 in office B. As shown in Figs. 6 and 9 and Figs. 10 and 13, the channel including repeating coils 152 and 252 and the channel including repeating coils 172 and 272 are associated with trunk circuit TRKltll in oliice A and with trunk circuit "TRK201 in oce B. The channel including repeating 3. 141 and 241 provide three two-way composite signaling channels between oices A and B. These signaling channels and associated signal relays are shown in detail in the right hand portion.. of Fig. 9- and left hand; Portion of Fig,A 1Q. One of these signaling channelscomprises theA upper conductor of trunk 70. and relays CX13 andl CX23.` and is. used for transmitting. supervisory signals betweenK trunkz circuitATRK1G1 in office A and trunk circuit-flRK,2,01 in oice B. Asecondone of these signaling channels comprises the lower conductor of trunk 50 and relays CX12 and `CX22 and isused for transmitting trunks-required signals. betweenI control circuit; 140 of oince A, and. control circuit 240 of ofiice B. The third signaling channelV comprises the upper. Conductor oi trunk 50. and relays CX11. and- CX21l and isused for transmitting control signalsl for synchronizingI the operating of commutator relays CM101 to CMIZQ of oilice A with the operation oi-feommutator` relays CM201- to CM220 of oiee B,y as. hereinafter described. Tghe commutator relays, which are shown` in Figs. 8 and.` 1.1, are, controlled by synchronizingl circuits, shown in Figs. 9` andy l0 andl comprising relays X1, Y1, CMCI, SIILSIZ; andy SL01 in office A and like relaysA in. oice B. The lower conductor of truuk 7 0 is used for neutralizing any difference in ground potential between. oiices AandB. Additional signaling channels, not shown,l are provided for. supervisory signaling between the other trunk circuits 13,0.4 and 230 in the same group.

Referring now to they reversible trunk circuit 130,` which is, designated TR;K1 01l and shown in-detail. inFigs. 3. and, 4. and, shown in part in Fig. 6 this trunk circuit is` a f Qllr-wire trunk circuit having one pair of talking conductors, identified as173 and 174. at the right end of Fig.i4, connected (as shown in Fig. land also in Fig. 9) to the talking channel of trunk 70for transmitting voice currentsfineoming from the associated trunk circuit 230 inoice Bg, another pair. of talking conductors, identied as153 and Y1;54, conuected (as shown in Fig. 1 andalso inf Eig. 9.). tothe talkingchannel of trunk 50. for trans-v m'ittingA voice currents outgoing to the. associated trunk, circuit-.230, inoice B; a signaling conductor M for. effect:v ing the transmission of supervisory signals over. the com: positesignaling channel comprising the upper conductor of trunkA 70. to, theJ associated trunk circuit 230 in oiiice B; anda signaling conductor E for receiving supervisory signalsover saidfcomposite signaling channel from the associated. trunk circuit 230. in oiice B. The conductorsA identifiedatA the left end of'Fig. 3 aS OS, OT, OR,O,T .1. and OR 1connect to terminals of crossbar switches 1,16, as. Shown. in Fig.. 1 for use. when trunk circuit T..RK1 0,1.

is in use as. an outgoing trunk circuit on calls outgoing;

tooilce B; and thel conductors identified` atthe, left end f; Fig.. 3. 21.5.1.5J IT,.IR,.I.T1, and IRLconnectto termi.-V nals of"crossbar switches 131 as .shown nFg.: 1; for use when trunk circuit TRK101 isY in use as.. an incoming trunkk circuiton calls incoming fromoice B'. Thecon ductorsV identified; at the lower` edge of Fig, 3 as.l KTI, KRl, KT, KR, T, R3Y03, 305, IS,ST, C and CLO are, as indicated, connected to a,` senderA link and connector circuit 117 as shown in Fig. l. The conductor MS at the top of Fig. 3 connects toa trunk block connector 121 as shown in Fig. l, ground being connectedA to conductorV MSwhenthe trunk circuit is busy or unavailable for use as.an outgoing trunk circuit. The conductors 101a and 10,1b, atthe top o f Fig. 3,A connect the resistor A101 across normally open contacts of'relay S, as shown yin Fig. 6; The conductors REL and O P, at the top of Fig. 4; are connected to contactsv of commutator relay vCM1'01 as shown in Fig. 8;

During periods when the trunk circuit TRK101' is availablefor useI only as an incoming trunk circuit, relay Sv will have been operated; as hereinafter described. With relay S operated, the trunk. percentage bridge resistor A101, is short-circuited. and ground is connected to the MS conductor to simulate a busy condition thereby making this trunk circuit unavailable for use as an outgoing trunk circuit. Relay S is locked operated until such a time as the trunk circuit is to be made available for use on calls outgoing to office B, as hereinafter described. Assume now that the trunk circuit TRK201 in office B, with which the vtrunk circuit TRK101 in oice A is interconnected, is seized on a call from oice B to office A, the signalV transmitted over' the composite signaling channel comprising the upper conductor of trunk operates relay CX13 shown in Fig. 9;; and relay CX13 connects ground to. signaling. conductor E of trunk circuit TRK10I, as shown in Fig. 9. The ground. thus connected to conductor E is extended through a back contact of relay OSI, inner left back contact of relay OS, a back contact of relay CO, and another back contact of relay OS to the winding of relay LC, thus operatively energizing relay LC. The ground connected to. Conductor.- E is. further extendedl from4 said; inner left back. Contact ofY relay OS.,A through another back, contactv of relay CO inparallel with. aV back contact of relay G to the. uppen winding of relay SVP, thereby operatively energizing relay. SVP- The operation. of relay LC closes a circuit. for operating; relay G.; short-circuits condenser TMC. to prevent; false firing ofy gas discharge tube TM; connects ground through theleft, back contact'v of relay CO.. to condnctorwSll leading to the sender link and. connector. circuit, thereby toeiect the connection of a sender 1118 for; use withthe trunk. circuit TRK101; and connects ground tov conductor OS, this` connection. beingl of no utility whileA relay Sis, operated. The operation of relay SVP., disconnects they outgoing` signalingv conductor M. from ground at a back contact of relay OS and connects this conductor to battery ata diiierent baclc. contact of relaywOS,4 thereby to eiect the sending of a stop sig- Ital'V over..thesignalingchannel tooice B thereby to prevent the, sending of, oliice. code and directory digit signais; to trunk circuit TR-K101 until, a sender 118 has, been attached theretofto. receive from oice Brmultifrequency. tone? signalsI representing the directory number of the called subscriber line. 'l'.heaforementioned operation.of, relay, Gf4 closes. a locking circuit for. itself, closesaA circuit. path` for maintaining the energization'. of relay LC after the aboye tracedzoperating circuit is opened by. the operation of relay CO, and connects groundto con.- ductor 310.

When. a sender isattached to trunk circuit TRK101, ground. is, connected in the sender to conductor 305, opperatingrelayv CO. Relay. CO, locks to grounded conductorV 3,10; connects ground, to conductor IS leading` to, the.. attachedY sender.; opens the operatingv circuit of. relay LC.;I opens the connection. between.. signaling. conductor E andthe upper winding of relay SVP, the. energization of this winding being maintained under. control of the associatedsender byfits` connection insimplex through the. windings. ofV retardation coil C. and. conductors. T and.. R. leading. to. the.; associated. sender; and. disconnects. grOund; fromA conductor SI leading. to. the, sender.. When` the senderiiasompleted., classfregistration and. a` test. for,

ground onconductorIS, .the senderclosesacircuit through.

conductor, 303.1for. operating relay SP. Relay Slop er. ated,b connects talkingconductors 153, and 154' of the trunk to conductors. T- and. R, leading, to the. senden.

connects conductors 1T and'IR from the. incoming link and connector circuit to conductors KT. and KR. leading to the sender; connects conductors ITI and l'Rl'from thev incomingJ link and connector circuit. to conductors KTl' andKRl) leadingtothesender; and' disconnects ground from conductor ISleading to the sender and to the incoming link and connector. circuit. When thesend'er is ready to receive tone signals representing the directory number of`the called line, it opens the simplex circuit over conductors T and Rhcausingrelease of' relay SVP'.

The release oirelay SVR disconnects battery fromand.

connectsA groundto, outgoingy signaling4 conductor. M' to cause transmission of'a "start`signal to oce B; where- 'tipnthe tone signals are transmitted from oice B over the trunk 50, through conductor 153 and 154, condensers 353 and 354, front contacts of relay SP, and through conductors -T and R to associated sender. When the sender has received the routing information, it obtains connection to a marker which controls the establishment of a connection from trunk circuit TRK101 through switches 131, 132, 135 and 136 as shown in Fig. 1 to an outgoing trunk circuit 137 which may be an intertoll trunk or a switching trunk, depending upon the location of the central oice in which the called line is located. After the sender completes the transmission of routing information, if needed, and the called line number, and before the sender is released, it disconnects ground from conductor 303, releasing relay SP. The release of relay SP reconnects ground to conductor IS leading to the incoming link and connector circuit to maintain energization of the hold magnets, not shown, of the switches 131, 132, 135 and 136 to maintain the connection therethrough with the outgoing trunk circuit 137. The release of relay SP also completes the talking connection from conductors 153 and 154, through condensers 353 and 354, back contacts of relay SP, conductors IT1 and IRI and the contacts of the crossbar switches to outgoing trunk circuit 137; and completes the talking connection from conductors 173 and 174, through back contacts of relay OSI, condensers 373 and 374, back contacts of relay SP, conductors IT and IR and the contacts of the crossbar switches to outgoing trunk circuit 137.

If the call is completed and the called subscriber answers, a circuit is closed from ground in the outgoing trunk circuit 137, through conductors of the crossbar switches and conductors IT and IR of the incoming link and connector circuit in parallel, back contacts of relay SP, windings of retard coil 375 and windings of relay R in parallel, thence through a front contact of relay CO and the upper winding of relay SVP to battery; whereby relay SVP is again energized to transmit an answer signal to oice B by disconnecting ground from, and connecting battery to, signaling conductor M. Relay SVP is under control of the called subscriber and, when the receiver is replaced on the switchhook, relay SVP releases to effect the transmission of a disconnect signal to oil-ice B. When the connection is releasedV from the calling end, the signaling relay CX13 is released whereby ground is disconnected from conductor E releasing relay LC. With relay LC released, relay IR is operated and renders eiective a circuit for charging condenser TMC, this circuit being traced from battery through ballast lamp BL, a front contact of relay G, resistors TMY and TMX, condenser TMC and resistor TMR, to ground. When the charge on condenser TMC reaches a value where the diiierence in potential across the control gap of tube TM is suicient to cause discharge thereacross, the tube is energized and the current in the circuit including the'main gapl and the right winding of relay G causes the lrelease of relay G due to the opposing energization of its two windings. The release of relay G opens its locking circuit, causes the release of relay IR, causes the release of relay CO, causes the deenergization of tube TM, and disconnects the winding of relay LC from signaling conductor E. The release of relay CO reconnects the windings of relays LC and SVP to conductor E awaiting seizure on another incoming call.

p During periods when the trunk circuit TRK101 is available for -use als an outgoing t-runk circuit, relay S will be non-operated so that -there is no ground connected to .conductoi MS lleading to .the trunk block .connector circuit except when this trunk circuit is busy on a call outgoing to oce B. Assume now that an incoming trunk circuit 1.10 in Fig. l receives a call, the completion of which under .the control of a connected sender 11'8 and marker 120 involves extension of the connection from trunk circuit -1-10 through switches 111, 112, 115 and 116 to an outgoing trunk circuit 130 which we will further assume is trunk circuit TRK101 shown in detail in Figs. 3 and 4.` When the marker 120 tests trunk circuit TRK101 through .trunk block connector 121 to determine its availability. for use in extending the connection to oice B, and this trunk circuit is then idle, ground is `connected lthrough a low resistance inthe marker, through :conductor MS, the right back contact of relay S, continuity back contacts of relays CO and G, a back contact of relay OSI and a lback contact of relay LC to the winding of relay OS, thereby operating relay OS. When switches 111, 112, `11'5 and 116 are operated under control of the marker to connect trunk circuit to trunk circuit TRK101., ground is connected in trunk circuit 110 through contacts of the switches -to conductor OS to maintain the operation of relay OS after the marker has completed its operations and been disconnected `from these trunk circuits. The oper-ation of relay OS disconnects incoming signaling conductor E from the windings of relays LC and SVP, connects ground to condenser TMC to prevent breakdown of tube TM, closes la circuit for operating rrelay G, and disconnects .the winding of relay LC from the winding of relay SVP. The operation of relay OS also disconnects ground from, and connects battery to, outgoing signaling conductor M to effect transmission of a seizure sign-al to oilice B. The operation of relay G .closes a locking circuit for itself and closes a locking circuit .for relay OS which includes the .continuity back contact of relay CO, thence over conductor OS and through the contacts of switches 116, 115, 112 and 111 to lground in the trunk circuit 110. The operation of relay G connects ground to conductor 310 thus causing the operation of relay O'Sl. Re'lay CS1 locks to conductor 310; opens the connection .between talking con-v ductors 4377 and 378 and conductors I1'73 :and `174; opens the short circuit across resistor TMX; and connects incoming signaling conductor E .through .a back contact` of relay SL .to battery.

When .the seizure si-gnal is received in oi-ce lB by the incoming trunk circuit 230 .therein associated with trunk cir-cuit TRK101 of ofce A, .an off--hook signal is transmitted therefrom as a stop sign-al which effects the operation of relay CX13, whereby ground is connected to conductor E to prevent the sender 118 -associated with trunk circuit TRK101 from starting the .transmission of tone signals representing the office code and numerical digits of `the called subscribers directory number to the trunk circuit 230 in office B. With .the sender 118 ready to transmit the ltone signals, ground is connected through a high resistance in the sender, through trunk circuit 110 and contacts of switches 1111, 112, 115 and 116, and through conductors OT and OR in panallel, back contacts of relay SP, conductors 357 and 3518, the windings of retard coil 355, back contacts of relay IR, a front contact of relay OSI, `continuity back contact and Winding of relay SL to battery. Being marginal, relay SLl does not operate; but a relay in tlhe sender operates, thereby effecting a reduction in the resistance included in this Icircuit to cause the operation of relay SL.- Relay SL, operated, disconnects its winding from, and connects signaling conductor E through resistor F to, the labovetraced connection to ground in the sender 118; and at the same time relay SL locks in parallel with relay OSI under the control of relay G. The operation of relay `SL lalso reconnects the talking conductors 173 and 174, through condensers '373 and 374 and conductors 377 and 378 to conductors OTl and ORl, thence to incoming trunk circuit 110; so that the outgoing talking path between trunk circuit 110 and conductors 173 and 174 is .complete and tone signals from :sender 118 may be transmitted thereover to the trunk circuit 230 in ofce B. When a sender 218 is .attached to trunk circuit 230 and this sender is ready to receive the tone signals, a start signal is transmitted iirom the trunk circuit 230' which causes the release of relay CX13, whereby ground is disconnected from signaling conductor E thus indicating to the sender T18 that the 'tone 'signals 'should ynow be transmitted. When the tone signal transmission 'is completed, the sender A1'1J8lis disconnected. Upon completon ofthe connection through yoce B to the called subscriber line -and answer of the call, the trunk circuit 230 in otice B transmits Aan oft-hookanswering'supe-rvisor-y `signal causing the Aoperation of relay CX13; vland ground is thereby connected to conductor E, 'thence through the 'above-'traced 'connection including lconductors 357 and "35S :to lthe incoming trunk circuit 110. `When the called Vsubscriber replaces the receiver on the switchhook, trunk :circuit 230 transmits an on-hook signal causing release 'of relay CX13 and *disconnection o'f ground 'from conductor E.

"When the originating toll operator Aor calling 'subsciber disconnects at *the end of conversation, the incoming trunk vcircuit 110 'disconnects ground 'from conductor fO'S causing the release 'of relay "OS and the -opening 'of the connections :between ltrunk circuit z1'10 and trunk `circuit TRK101. 'The release 'ofrelay OS reconnects mground 'toconductor OS and 'to 'conductor MS as a busy indication; A'opens the 'operating circuit Yof relay Gdisconnects battery from,and connectsground to, conductoriM to xeiiect the transmission of a disconnect signal to trunk circuit 230 in office B; and opens the shortoi-rcuit'ing connection across condenser TMC, whereupon this condenser is charged through timing resistors TMX ia'nd`TMY in series. When the potential difference across the control gapof tube TM resulting from rcharging condenserT/MC, causes breakdown of thetube, the energization Iof the'right winding of relay 'Gby the current in the main gap causes release of `relay'G followed by release of relays OSI vand SL. y'The release of relay 'G opens its flocking'circuit, and'de-energizes tube TM. The release 'of relays OSI 'and SL restores trunk circuit 'PRK-101-to-normalincluding the disconnection-of ground -from conductors MS and OS whereby `the trunk circuit may now be seized for use ias `an outgoing trunk circuit o'n another call.

The direction reversing control circuit 14001? oice A is -shown in detail in Figs. to 9, inclusive; and 'the like circuit 240 of-cilice `B `is 'shown in Figs. l0 to 14, inclusive. The circuit unit 140 is common to the group of twenty reversible trunk circuits 130 in otlice A', and'the control-circuit 240 is common 'to the associated group of reversible 4trurik circuits l230 in oce 1B.

The `trunk reversing-control equipment in office A includes a -timer circuit, shown in Fig. 5, having a synchronous motor-driven switch 125 which maybe common to fa'll of the groups of reversible ltrunksin the cnice. The

switch has a contact 'segment for each l5-rninute period of fthe-day,^eachsegment'being engaged by wiper 126 `for two minutes out of the l5-rninute period. A cam 127 on Ithe motor Vshaft closes 'a `contact 128 for two seconds in every 2-minute period. Cross-connecting terminal means `129 'facilitates connection 'of the switch Asegments engaged by wiper 126 Ato efectthe operation of any vdesired Aone of a group of time pattern relays TP100 to TPI-20, inclusive, shown in Fig. 5, there being a group of these 'relays foreach group of reversible 'trunks in oice A. The'singl'e fgroup oftimepattern relays, shown in Fig. 5, is associated with the reversible trunks between oces A l'and B. Each one of these relays corresponde-'to lthe percentageof these trunks required to carry the -outgoingtrafic to oiiice B fduring the intervals corresponding Vto fthe-'timer switch segments to which the Vrelay is operatively connected, lthere being one relay for-each percentage "fromO-percentlto Y100 percentin '5 percent increments. 'The operation of any of time pattern relays TP101 'to VTF1-20 closes 'acircuit for operating fthe-correspending Tone Tof-'a igroup of -twenty trunk percentage relays l2101 to T120, shcwnin Fig.6. The operation of any one of time pattern Irelays TP100 4to T9120 connects ground :to the 'associated percentage conductorto t'shortcircuitfthe holding 'winding of .any foperated, higher numtit) bered trunk percentage relay and thus cause rits release. The'opera'tion of any one -of the time pattern 'relays 'also opens la chain connection, thereby disabling lthe trunk reversing control for eac'h 2second period 'during which :a time pattern-relay `is operated. The operation of any one of 'the trunk percentage relays causes'the successive foperation vof each lower numbered relay, .if 'notalready operated.

The trunk percentage relays together withithe S relay in each 'of l:the :twenty trunks TERK-101 to TRK120 partially shown in Fig. 6, control the balance of a resistance bridge across which the windings of .polarized .relays T-A-10 and TR10 are connected. fOne arm of the bridge includes resistors R101 1tofR`1120, inclusive, each of which `may be short-circutied by 'back .contacts of the .associated one of relays T101 'to T120, inclusive; and another Aarm ofthe bridge includes `resistors .A1 `to A20, inclusive, Xeach of which may lbe short-circuited'by front contacts of the S relay in the Vassociated `one of trunk circuits' TRK101 to TRK120. Wfhen .the number of trunksavailable, as indicated .by the non-operated condition of relay S in each, equals 'the number of :trunks required as 'indicated by the operated condition ofthe trunkpercentage relays T101 to T120, the bridge .is balanced and neither relay '1`A1'0 nor`TR10-ioperates. When -the number of trunks required for carrying the outgoing tralic from oilice A to office B is larger I"than `the num-ber of trunks available, `the 'trunksrequired ,relay TRlt) operates; and, when the number of trunks available is larger than the number of trunks required, the trunks-available relay TA10 operates. ,Relays TAM, 'TA-12, TRM and TR12, shown in Fig. 7, control the transmission of ltrunks-available and trunks-required signals over the signaling channel comprising the lower conductor of trunk 50; and :relays TKG-11 -and TRG12 initiate -the :reversing loperation when all trunks in the group available for useasoutgoing trunks are busy.

The commutator relays CM'10'1 -toCMi120 in-ofiiceA sequentially and cyclicallyconnect the winding vof the S relay ineach .of the reversible trunk circuits for operation or release whenever the `direction of any of the `trunk circuits lis to be reversed, as lindicated vby an unbalanced condition of `the resistance bridges comprising resistors R101 "to R120 yand A101 to A120. And -to synchronize the operation offthecommutator relays CM101 to-CM120 in oflice A with :the 1'operation of the commutator .relays CM=201 Vto CM220 in olice B, synchronizing signals are transmitted yover the signaling channel comprising relays CX11 .and CX21 Vand the `upper conductor of .trunk 50. The commutator relays are operated -in a chain, .each -relay Voperating under lthe .control of the preceding relay and .locking under .controlof .the succeeding relay in the chain vas :hereinafter further described.

Whenever relay TA10 is operated, because there are more trunks available .for use -as outgoing trunk circuits in :oceiA than the-operated condition ofthe trunkrpercentage relays .indicates are required, it closes a circuit for operating .relay TA11, thereby connecting Vbattery through Vconductor '165,retard coil 166, upper winding of relay CX12, .lower winding ofretard coil 151,.lower conductor Yof trunk 50, lower winding yof retard coil A251, middle and upper windings of relay CX22 to ground, causing the operation of relay CX22 in oilice B; and wheneverrelayTAZ() is `operated due to a similar condition in othce B, a trunks available signal is transmitted over the lower .conductor of trunk operating relay CX12. Whenever'relay TR10 is operated, because there are Ifewer trunk circuits available in otiice A 'for use as outgoing trunk circuits than 'the percentage relays indicate should be available Ifor such use, relay lTRM) vcloses a circuit for operating relay TRU. -The operation of relay TR11 closes a circuit v'for'operating relay TR12'provided relay CX12 is at that time operated responsive to a trunks-'available signal incoming 'from 'oice B; 'and transmits a trunks-required lsignal by 'connecting ground to conductor '165 iso that rrelay CX22 in o'ice B is non-operated. Whenever relay TITRTZG 'isoperated, .because 'there 'are fewer 'trunk-'ciraree-,8e4

cuits" available otr-lee' B fori use as outgoing' trunk cir cuits than the percentage relays` indicate should be avail v able for such use,- relay TR21- is operated;- and, with. relay `CX22 Ithen operated responsive to atrunks-available sig-nal incoming trornoiliceA, relay TR2`2- is operated. The operation of relay "EIR-2 transmits atrunks-required signal toI oilice A wherebyrrelay CX12 in oliice A is nonoperated. It the resistancebridges in both offices are balanced, noneof the trunks-available and* trunks-required relays are operated.l At sucha time, sorne one of the commutator relaysineachl oli'ce is locked operated under control of the next higher numbered cornutator relay; andY relays' STM, ST12, SL01, and CM-Cl in olice A and lthe corresponding relays inoice B' are not operated. Relays X1` and Y1 in. ofce A are controlled by relay CX11 andi relays X2 and Y2 in office B are controlled by relay CX12 toconnect the windings of relays CMCl and CMC2 tothe windings ot the eomrnutator relays in succsession whenever' trunk circuitsare tobe reversed as to the direcitio'n` otcalls therethrough.

Assume now that the resistance bridges become unbalanced, more trunk-s being' required for use as outgoing trunks in oice A, relays TRM), TR11 and TR12 being operated as above described to establish the trunksr'erpliredY signal condition, whereby relay CX12 remains noneopera-ted.` The operation of relay TR12 closes a circuit for operating start relay ST11 of the synchronizing circuitshown in Fig. 9; prevents closure of the operating .circuit of relay TA12; and prevents closure of the circuit for operating' trigger relays rERG11 and TRGIZ. The operation of sta-rt relay ST1-1 closesV a locking circuit through its upper winding,l closes ai circuitfor operating relay ST12 and closes a circuit for operating relay SL01. The operation of relay ST12 disconnects ground from, and connects battery to; conductor 155, thence through retard coil 156, upper winding of relay CX11, upper winding of retard coil' 151, to the upper conductor of trunk 50 tooperate relay CX21 in olice B. r.[lhe operation ofrelay SL01 closes a locking circuit through its upper winding and opens the operating circuit of relay STI. Assuming further that relays X2 and Y2 are nonoperated at the tirne relay CX21 operates, the operation ot relay CX21 closes a circuit for operating relay X2 of the synchronizing circuit in oflice B. The operation of relay X2 closes a circuit including the winding of relay CMC2, front contact of relay'XZ, back contact ot relay Y2, front contact of the operated one of the commutatr relays, for instance relay CM2-18, and the winding of the adjacent succeeding cornmutator' relay, for instance relay CM21`9, whereby relay CMC2 and 'the cornrhutator relay are operated. The cornmutator relay thus operated, for instance relay CMZ19, locks to the next succeeding relay and causes the release of the adjacent preceding relay; and the release of this preceding comrnutator relay causes release of relay CMC2. The aforementioned operation of relay CMC2 disconnects ground from, and connects battery through, conductor 255, retard coil 256, upper winding of relay CX21, upper winding of retard coil 251, to the upper conductor of trunk 50 to operate relay CX11 in otlice A; and the aforementioned release of relay CMC2 disconnects battery from and reconnects ground to the upper conductor ot trunk 50 to cause the release of relay CX11 in oice A.

Assuming' relays X1 and Y1 to be non-operated at the time that an operation ot` relay CMC2 in otlice B causes the operation of relay CX11 in otlice A, the operation of relay CX11 causes the operation of relay X1. The operation of relay X1 closes a circuit including the winding of relay CMC1, front Contact of relay X1, back contact of relay Y1, front contact of the operated one of the commutator relays, for instance relay CM118 and the winding of the adjacent succeeding commutator relay, for instance relay CM119. Relay CMCl and the commutator relay are thereby operated. The commutator relay thus operated, for instance CM119, locks to the adjacent succeeding relay andi' causes the release of. the adjacent preceding relay; and the release ot this preceding relay causes the release of relay CMCl. The aforementioned operationk of relay CMCl causes the release of relays STH, ST12 and SL01, the cornrnutator rela-ys ineach office having been advanced one step. When relay ST12 releases, ground is reconnected to conductor causing release of relay CX21 in oilice B.

The release of relay CX21 :after operation of relay X2, closes a circuit for operating relay Y2, relay X2 being held operated by its upper winding; and the release of relay CX11, after operation of relay X1, closes ya circuit for operating relay Y1, relay X1 being held operated by its upper winding. Assuming that relays TR10 and TR11 remain operated, the release of relay SL01 causes the reoperation of relay ST11. Relays ST12 and SLO1are thereby reoperated. The reoperation of relay ST12 'again disconnects ground from, and connects battery to, conductor 155 to reoperate relay CX21 in oice B. The reoperation` of relay CX21 connects ground to conductor 258, thence through a front contact of relay Y2 to shortcircuit the upper winding of relay X2, causing the release of relay X2. With relay Y2 operated,` the release of relay X2 closes a circuit for operating the next commu-- tator relay in office B and relay CMC2 in series. The reoperat-ion of relay CMC2 disconnects ground. from, and connects battery to, conductor 255 thereby causing the.` reoperation of relay CX11 in oice A. The reoperatiorr ot relay CX11 connects ground to conductor 158, thence through a front contact of relay Y1 to short-circuit and thereby cause the release of relay X1. With relay Y1 operated, the release of relay X1 closes a circuit for operating the next commutator relay in office A and relay' CMCl in series. The newly operated commutator relay' in each oiiice locks under control of the adjacent succeeding relay :and causes release of the preceding relay and release ofthe associated relay CMC2 or relay CMCI.. The release of relay CMC2 aga-in. causes the release of relay CX11, and the release of relay CMCI again causes the release of relay CX21, in the mann-er hereinbefore described. The release of relay CX21, after both of relays X2 and Y2 have been operated, short-circuits the locking winding of relay Y2 to release relay Y2, thereby completing another step in the cyclic operation of the cornmutator relays in otiice B; Eand the release of relay CX11, after both ofV relays X1 and Y1 *have been operated, shortJ-circuits the locking winding of relay Y1 to release relay Y1, thereby completing another step in the cyclic operation of the commutator relays in oice A.

With the trunks-available condition existing in ofce B and a trunks-required condition existing in oice A, the operation of any commutator relay in otlce B closes a connection from ground through back contacts of the time pattern relays, front contact of relay TA22, front contact ot the commutator relay, to conductor OP of the associated trunk circuit so as to eiect operation of the S relay if this relay is not operated and if the trunk circuit is lidle as indicated by the non-operated condition of relays LC and OS. As hereinbefore described, operation of the S relay of a trunk circuit renders it unavailable for use Sas an outgoing trunk circuit. At the same time the operation of the corresponding commutator relay in oice A closes 'a connection from ground through back contacts of the time pattern relays, front contact of relay TR22, front contact of the commutator relay, to conductor REL of the associated trunk circuit so as to effect the shortacircuiting of the S relay in this trunk circuit and cause release of this relay, thereby to render the trunk circuit available for use as 'an outgoing trunk circuit. As soon as the required number of trunk circuits are available in eac'h oice for use as outgoing trunk circuits, the resistance bridges in the two offices are balanced and the trunks available and trunks required relays which were operated release; and further operation of the commutator relays does not occur until the trunk percentage bridges again become unbalanced. f The trunk reversng operations which occur when additional trunk circuits are required in office B for use as outgoing trunk circuits are similar to the above-described operation occurring when additional trunk circuits are required for use as outgoing trunk circuits in oice A.

If at any time, an incoming trunk circuit such as trunk circuit 110 or 210 is seized and at that time all the trunk circuits available in that oce for use as outgoing trunk circuits are found busy when the marker tests the group, the TSD1 or TSD2 relay is operated. Assume relay TSD1 to be operated due to such a condition in oice A, thereby closing a circuit for operating relay TR11. The operation of relay TR11 connects ground to conductor 165, hence through retard coil 166, upper winding of relay CX12, lower winding of retard coil 151, lower conductor of trunk 50, lower winding of retard coil 251 in oce B and the upper and middle windings of relay CX22 to ground. Relay CX22 thereupon closes a circuit for operatively energizing the lower winding of relay TRG21 in oice B, which circuit includes a back contact of relay TRG22 and the contacts of interrupter INT2, back contacts of relays TR21 and TA21 to ground at the back contact of relay CX22. The interrupter contacts are cyclically closed and opened once each second, the closed period being for .5 second. The operation of relay TRGZI closes a circuit including the interrupter contacts for operating relay TRG22. Relay TRG22 locks under control of the interrupter and relay CX22, opens the operating circuit of relay TRG21, and

vextends the ground connection including the interrupter contacts through the upper winding and upper front contact of relay TRG21 to shunt the winding of the highest numbered trunk percentage relay which is operated. When this trunk percentage relay releases, relay TRG21 releases but relay TRG22 remains locked until the interrupter contacts are opened, the trunks-required signal from office A is terminated, or relay TA21 or TR21 operates. The release of one of the trunk percentage relays unbalances the resistance bridge in oflice B to operate relays TA20 and TA21, whereupon the commutator relays in the two ofces are operated in the manner above set forth'to reverse the direction of trunks until the resistance bridges are again balanced. If at any time, both the TSD1 and TSD2 relays are operated, both of relays TR11 and TR21 are operated and the trunks-required signal is transmitted from each ofce to the other, in which case no further reversal of trunks occurs until the TSD1 relay or the TSD2 relay releases to terminate the trunks required signal from one of the oces.

What is claimed is:

1. In a communication system comprising a group of interoice trunks, each trunk adapted for use on a call in either direction, a trunk circuit at each end of said trunk arranged forruse either as an incoming trunk circuit or as an outgoing trunk circuit, relay means in` each trunk circuit for marking the trunk circuit unavailable for use as an outgoing trunk circuit, means in each office determining at intervals the number of trunk circuits in the office to be marked unavailable for use as outgoing trunk circuits, and means in each office for selectively operating or releasing said relay means of one or more trunk circuits in succession to effect the marking of a dilferent number of trunk` circuits as unavailable for use as outgoing trunk circuits.

2. In a communication system comprising a group of interoliice trunks, each trunk adapted for use on a call in either direction and having a trunk circuit at each end thereof arranged for use on a call in either direction, circuit means in each trunk circuit operable to mark the trunk circuit unavailable for use as an outgoing trunk circuit, other means in each trunk circuit for marking said trunk circuit busy when in use as an outgoing trunk cirf were previously available.'

cuit, timer means in each office, lregister means in each-A l office variably settable under control of said timer means to register the number of trunk circuits to be marked in each oflice as unavailable for use as outgoing trunk circuits, control means in each office for selectively operating or releasing said circuit means of said trunk circuits, and means in each office jointly controlled by said register means and said circuit means of each trunk circuit for selectively connecting said control means to each trunk circuit one at a time in succession.

3. In a communication system comprising two oces and a group of two-way trunks therebetween, a trunk circuit in each otlce for each of said trunks, each trunk circuit having circuit means therein operable to mark said trunk circuit as unavailable for use as an outgoing trunk circuit, timer means in each oice, register means in each oflice variably settable under control of the timer means to indicate the number of trunk circuits to be marked as unavailable for use as outgoing trunk circuits, control means in each office for selectively operating said circuit means of said trunk circuits, and commutator means in each ollice rendered effective by the marking of a number of trunk circuits as unavaliable for use as outgoing trunk circuits diterent from the number indicated by the setting of the register means for connecting said control means to each trunk circuit in succession.

4. In a communication system comprising two otiices and a group of two-Way trunks therebetween, a trunk circuit in each otce for each of said trunks, each trunk circuit having circuit means therein operable to mark said trunk circuit as unavailable for use as an outgoing trunk circuit, timer means in each oilice, register means in each office variably settable under control of the timer means to indicate the number of trunk circuits to be marked as unavailable for use as outgoing trunk circuits, control means in each ottice for selectively operating said circuit means of said trunk circuits, commutator means in each oice rendered effective by the marking of a number of trunk circuits as unavailable for use as outgoing trunk circuits different from the number indicated by the setting of the register means for connecting said control means to each trunk circuit in succession, and means including signaling means between said oices for synchronizing the connection of the control means to the trunk circuits in one ofce with the connection of the control means to each corresponding trunk circuit in the other office.

5. In a communication system comprising two ofces vand a group of two-wayl trunks therebetween, a trunk circuit in each oice for each of said trunks, each trunk circuit having circuit means therein operable to mark said trunk circuit as unavailable for use as an outgoing trunk circuit, timer means in each otce, register means in each otlice variably settable under control of the timer means to indicate the number of trunk circuits to be marked as unavailable for use as outgoing trunk circuits, control means in each otlice for selectively operating said circuit means of said trunk circuits, commutator means in each oce rendered effective by the marking of a number of trunk circuits as unavailable for use as out- Y going trunk circuits different from the number indicated by the setting of the register means for connecting said control means toeach trunk circuit in succession, and means effective when all trunk circuits available for use as outgoing trunk circuitsin one otlice are busy for changing the setting of the register means thereby to start the operation of said commutator means and thereby render available in said oice a larger number of trunk circuits for use as outgoing trunk circuits than 6. In a communication system comprising two oces and a group of two-way trunks therebetween, a trunk circuit in each office for each of said trunks, each trunk circuit having circuit means therein operable to mark said trunk circuit as unavailable for use as an Aoutgoing trunk circuit, timer means in each oiice,l register means in each oice variably settable, under control of the timer means to indicate thenumber of trunk circuits to be marked as'unavailable for use as outgoing trunk 

